Watch out for old tile adhesive under that vinyl plank

I pulled up some old sheet vinyl in a Phoenix kitchen last week and found a thick, dried layer of black cutback adhesive. I thought I could just scrape it, but that stuff was like rock. I spent over 8 hours with a 4-inch scraper and a heat gun just to get the floor flat enough for the LVP. Has anyone found a faster way to deal with that old mastic without damaging the subfloor?